Record ionisation of Xenon
An X-ray laser removes more than two complete shells from the electron cloud surrounding noble gas atoms
Atoms have been thoroughly researched, but they are still good for a surprise. Researchers of the Max Planck Advanced Study Group (ASG) at the Hamburg Center for Free-Electron Laser Science (CFEL) have now produced atoms of the noble gas xenon with an extraordinarily high charge. With only one flash of the world’s most powerful X-ray laser, an international team headed by Daniel Rolles ejected 36 electrons from a xenon atom and thus stripped the atom of more than two of its highest energy shells. This record charge significantly surpasses the greatest possible ionisation which the physicists estimated for the X-ray energy used. This only becomes possible due to a resonance effect, which the team discovered. In the future, researchers will have to take into account these results when they use the extremely powerful pulses to bombard proteins or other biomolecules, for example, whose structure can only be clarified with the aid of an X-ray laser.
If an apple tree were like an atom, a fruit farmer would have a difficult job. When picking the fruit, he would have to apply more force for each further fruit, and picking the third dozen would already mean he had to pull 100 times harder on an apple than for the first apple. Farmers are spared such effort - but not atomic physicists: the more electrons they remove from an atom, the more energy they have to apply to remove the next one. Help is now on the way in the form of a previously unknown effect. This allowed a collaboration headed by the physicist Daniel Rolles from the Max Planck Advanced Study Group at the CFEL to produce xenon with a 36-fold charge with only a single pulse of an X-ray laser, although they thought that, theoretically, only 26-fold ionisation would be possible at the photon energy used.
For their experiments, the researchers used the pulses of the X-ray laser at the Linac Coherent Light Source (LCLS) of the US research centre SLAC in California. The light particles (photons) of the X-rays used contained around 1.5 kilo-electronvolts, i.e. one thousand times more energy than visible light. The laser pulses also had some other extreme characteristics, however: each flashes for a mere 80 femtoseconds, which is less than one hundred thousandth of a billionth of a second, and transports around a thousand billion photons. For comparison: this roughly corresponds to the intensity if all the sunlight reaching Earth could be focussed onto an area the size of a thumbnail.
Several photons eject electrons from the atom
If a photon of such an X-ray flash strikes an electron in the xenon atom, it transfers its energy to the electron. Depending on how tightly bound the electron is, this collision can push it out of the electron shell. Since the light particles have such extremely high energies, this would apply per se to several electrons. In addition, the energy boosts perturb the complete electron shell of the atom, which makes it possible for further electrons to be shaken from the atom, so to speak. What’s more: the extremely high photon density of the pulses means many atoms are struck by several photons from a single X-ray flash in such a way that electrons are ejected from their shells.
All in all, it should be possible to remove a maximum of 26 of the 54 electrons in the noble gas with a photon energy of 1.5 kilo-electronvolts- at least this is what physicists used to believe. The conventional school of thought said the remaining 28 would be too tightly bound even for this photon energy. In reality, the scientists in Daniel Rolles’ group observed that up to 36 electrons were ejected from the atom. The X-ray flash therefore completely removed the two outermost shells and even a portion of the third shell of the noble gas’s five electron shells. These outer shells comprise the upper energy levels of the atom. “As far as we know, this is the highest degree of ionisation which has so far been achieved in an atom with a single electromagnetic pulse,” emphasises Rolles. “Our observation shows that the existing theoretical approaches must be modified.”
The cause for the ionisation once assumed to be impossible is a resonance: the xenon electrons can absorb a large amount of radiation in the energy range of the X-ray flashes. This causes some of them to be removed from the atom directly; others are transferred into an excited, i.e. higher energy, state, but are still bound. If one of the excited electrons falls back into its original state, however, energy is released again, which can provide the necessary extra kick to another excited electron in order to remove it from the atom completely. In rare cases, the already excited electron is hit by a second photon from the X-ray flash and thus ejected from the electron shell.
The xenon atoms absorb twice as much energy as expected
In xenon, this resonance effect is particularly pronounced at a photon energy of 1.5 kilo-electronvolts. Ultimately, this comes down to the fact that electrons cannot make just any energy jump in an atom; they can absorb only specific quanta of energy. Light particles with 1.5 kilo-electronvolts can excite electrons in the interior of the atom just so much that a further energy boost ejects them from the atom. Accordingly, the xenon atoms would not be as strongly ionised at a higher energy of two kilo-electronvolts as with the 1.5 kilo-electronvolts pulses.
“The LCLS experiment has produced an unexpected and unprecedented charge state by directly catapulting dozens of electrons out of an atom,” emphasises co-author Benedikt Rudek, doctoral student at the CFEL, who analysed the data. An atom with 36-fold ionisation absorbed at least 19 photons in this process, thus an energy of more than 28.5 kilo-electronvolts. “This is twice as much as we had expected,” says Benedikt Rudek.
The CFEL scientists used the measurements to refine a mathematical model which allows such resonances in heavier atoms to be calculated. The observations provide the physicists not only with new findings on the interaction between atoms and light, they also have a practical significance for the research: “Our results provide a recipe for maximising the electron loss in a sample,” explains Daniel Rolles. This can be desirable or undesirable. “Researchers who want to produce a plasma with high atomic charge states can use our results, for example.” Plasmas with such high charge states can be used for astrophysical experiments, for example.
Biological experiments should avoid the resonance region of heavy atoms
When investigating biological samples, on the other hand, scientists should avoid the resonance regions of heavy atoms. “Most biological samples contain a few heavy atoms,” says Rolles. If samples, such as proteins, are investigated with X-ray pulses in the resonance region, for example, the heavy atoms are particularly strongly ionised and the molecules fragment easily at these positions. This damage can harm the imaging quality.
The precision measurements at the LCLS were performed in an experimental chamber developed by the ASG at CFEL, which was shipped to California in a total of 40 crates . This CFEL-ASG Multi-Purpose Chamber (CAMP) was installed at the LCLS for three years and was used in more than 20 experiments. In addition to the CFEL, which is based on a cooperation between the Max Planck Society, DESY and the University of Hamburg, and the US research centre SLAC, the investigation involved researchers from the Max Planck Institutes for Nuclear Physics, Medical Research and semiconductor laboratory, as well as from around a dozen other institutions in Germany, France, Japan and the US.